Operational amplifiers, because of their versatility and predictability of operation, are extremely useful building blocks in both analog and digital circuit applications. Stability is generally provided in such amplifiers by external feedback so that variations in gain don't affect circuit performance. Dynamic adjustment of the gain of an operational amplifier is not readily achieved.
There are circuit applications, however, in which it is desirable to provide convenient and continuous control (attenuation or amplification) of output signals of an operational amplifiers while maintaining the stability of operation provided by fixed feedback circuitry. In one application, for example, the effective resistance of a switched capacitor circuit is controlled by varying the level of voltage which is available to charge the capacitor. One known way of providing controlled attenuation of the output of an operational amplifier is to use a bipolar circuit to convert the amplifier output voltage to a current and to employ a Gilbert Cell in which current is steered to a load resistor as a function of applied control voltage. Gilbert Cells are described in a book entitled "Analysis and Design of Analog Integrated Circuits", Paul R. Gray and Robert G. Meyer, 2nd Edition, Page 593, 1984, John Wiley & Sons. Such an arrangement, however, has serious technical complications such as inherent distortion problems in the required bipolar voltage to current converter and the need for a logarithmic differential control voltage.